There's not much of a diff between 26 & 27" tires.
Some brands 26 are taller than other brands 27"s.
The factory originals unless your running a ton of pressure (6lbs+) will likely only measure 23-1/2 or 24".
I personally like the 27" because I tend to run less than 4lbs for better ride, traction & floatation. They measure in at 26" in height.
There should be lots of clearance on the rear for either tire.
Should also be enough on the front. I see you quoted 9" front width, I think thats a good idea, it'll be slightly easier to steer that way.
I would think you will gain about 4-5mph in your cruise speed with 27"s.
The biggest gain you will notice is ground clearance, especially in ruts!
